The present invention relates to methods and apparatus for building, completing, repairing and performing similar operations on subsea pipelines, particularly those of large diameter.
The building and repairing of large diameter subsea pipelines, and the completion and connection of such pipelines to marine risers has been extremely tedious, cumbersome and expensive, primarily because of the individual pipe sections, connectors and other components of the pipeline system are large, and extremely heavy. A single section of 36 inch diameter steel pipe with a cement weight coating may weigh ten tons or more, so that manual positioning by divers is rendered impossible.
In the past, these operations have typically been carried out using pipe slings and cables supported from cranes carried by surface vessels to support the pipe sections and utilizing numerous divers to hand position the pipe sections held by the cranes. Such operations are, of course, adversely affected by weather conditions or rough water at the surface. In the North Sea, and similar areas plagued by bad weather, this method has been rendered nearly infeasible.
Past attempts at utilizing subsea cranes, pipe frames and the like for handling these components have met with only partial success, due to their inability to operate satisfactorily on the rough terrain encountered on the ocean floor and due to their inability to lift and reposition the enormous loads encountered with the required accuracy.
In repairing damaged subsea pipelines of large diameter, a further particular problem is encountered where the anchor, ship or other cause of the damage has deflected the pipeline from its normal course, so that after removal of the damages portion, the pipeline ends may not be rejoined using commercially available linear components. In some instances, connecting sections have been custom fabricated to rejoin the misaligned pipe sections, relying upon a diver's measurements and estimates in fabricating the connection, with attendant difficulties. In other instances, repositioning of the pipeline sections in linear relationship has been attempted, but this generally requires the use of surface cranes or derricks, no subsea apparatus having previously been available which can accurately reposition the pipeline ends and carry out the successive steps of installing tubular connections to rejoin the ends.